of activity, such as manufacturing or commerce. Some studies in advanced countries have tried to work out thresholds for various activities that appear only when cities attain a certain population. Unfortunately, there are few if any comparable studies of developing countries, but it seems certain that the results obtained in developed countries cannot automatically be applied to cities in developing countries. Almost certainly, the threshold for the appearance of many activities will be higher in contemporary developing countries. An independent but related problem is determining economies and diseconomies of scale of different city sizes. Few would argue that there is a direct relationship between size and overall efficiency. At some point the diseconomies stemming from high land rents, traffic congestion, pollution, etc. will come to balance out or outweigh the advantages of population concentration and high division of labor. So little hard evidence is available on this point that little can be said other than to note that differences between developed and developing countries may be considerable. These considerations come to a head in dealing with one prominent feature of the urban hierarchy, the position and role of the "primary" or first city of a country. Many developing countries are characterized by "high primacy"; the first city is many times larger than the secondary cities, and this demographic concentration reflects a high degree of concentration of governmental, economic, and cultural activities in one place in the country. There has been a tendency to condemn all cases of high primacy as being detrimental to economic development because they are "parasitic"—sucking out the best from the rest of the country and offering little in return. However, conditions vary among countries. As an example, two countries may demonstrate equally high primacy; their first cities are five times the size of the second cities. In one country a review of the situation leads to a recommendation that the pattern be maintained, whereas in the other country the recommendation is to use all means to reduce it. Why the contrary policies? The first country is small in population (say 5 million) and compact in area. Assuming a good transportation network, the primary city is within a few hours of all populated parts of the country. The advantage of maintaining high primacy and moving, in effect, toward a city-state pattern is that it will permit the existence of at least one genuinely metropolitan center that can provide the range of goods and services that a modern country requires. One city of a million inhabitants may be better suited to the country's needs than five cities of 200,000 each. The other country has 50 million people and encompasses a large irregular area; therefore high primacy may handicap regional development. The one center cannot well serve such a large and dispersed population. And the primary city would attain so large a population that it would result in local diseconomies. In short, in primacy variation just as in other aspects of urbanization, no simple formula will suffice.